The present invention relates to a camera for taking picture of the eye fundus and particularly to an eye fundus camera, which is equipped with a mechanism for illuminating the eye to be inspected without causing the dilation and the contraction of eye pupil.
Roughly speaking, there are two kinds of eye fundus cameras. One illuminates the eye with visible light to permit focusing. The other illuminates the eye with light such as infrared light which does not cause dilation or contraction of the eye pupil. In the first case it is necessary to apply a pupil dilating agent to the eye in order to avoid contraction of the pupil in response to visible light. However, this agent not only bothers the patient but also creates the danger of damage to the eye or of an infection such as latent glaucosis.
In eye fundus cameras which focus on the eye fundus in the naturally dilated state of the pupil, the eye is illuminated by infrared light and an image is formed on an infrared light image tube or the light sensing plane of an infrared video tube. The resulting infrared light image, which is equivalent to the visible light image of the eye fundus to be photographed on film, is electrically converted into a visible image and displayed. The operator focuses upon the eye fundus by operating focusing means on the basis of the sharpness of the thus produced image. However, the color of the eye fundus is such as to produce little contrast in response to infrared light. Furthermore if the eye fundus is inflamed due to an infection, it is difficult to obtain a clear image of objects such as veins. This makes focusing of the camera all the more difficult.
Another example of the eye fundus camera which focuses infrared light is disclosed in the U.S. Pat. No. 3,614,214 (Oct. 19, 1971). The eye fundus camera disclosed in this literature presents, beside the main body of the photographic system, an automatic optometer equipped with focussing means functionally engaged with the focussing means of the main body of the photographic system. This optometer presents a means for forming an infrared light beam alternatively directed along two different directions and a mark so as to project the image of the mark on the eye fundus by means of the two light beams. In case the image of the mark is out of focus, two images of the mark appear alternatively at the different places on the eye fundus. If the image of the mark is in focus two images of the mark appear alternatively the one over the other at the same place of the eye fundus. This image on the eye fundus, of the infrared mark is reprojected on two adjacent infrared photo detectors and the then produced currents are applied to a subtractor. The difference signal between the currents from both detectors, permits automatic control of the focussing means of the automatic optometer and therefore at the same time also the focussing means of the photographic system. The most important shortcoming of the above mentioned eye bottom camera is that it is impossible to determine the photographic view field during the focussing operation. In other words, it is impossible to confirm which part of the eye fundus is photographed. The shutter of the camera is automatically released, while the operator is not sure on which part of the eye fundus the image of the focusing mark is projected.